This invention relates generally to methods and compositions for controlling the tack of materials, and more particularly to methods for decreasing the tack of adhesives, pitch, and other particulate, dissolved or colloidal contaminants to minimize deleterious effects of these materials.
In the paper production industry, one area of increasing interest is the reuse of wastepaper; however, the removal of contaminants from the wastepaper or from process streams used to process the wastepaper is key to the ability to reuse this wastepaper. Many contaminants adhere to paper fibers, thereby causing problems during the recycling process. Two such contaminants are known as “stickies” and “pitch.” Stickies generally comprise materials originally used as adhesives, including, but not limited to, hot melts, pressure-sensitive adhesives (PSAs), latexes, and binders. Pitch is a natural component associated with both virgin and secondary fibers originating from extractives released from wood during pulping.
Contaminants may cause operational problems during the processing of wastepaper for reuse and may also reduce product quality. Specifically, contaminants may be deposited on wires, felts, press rolls, and drying cylinders of paper machines. In addition, contaminants in the papermaking process may hinder bonding of fibers and increase the frequency of web breaks. Consequently, contaminants must be rigorously controlled in order to improve papermaking operations and product quality.
Tack describes the adhesive property of contaminants. By reducing the tack of contaminants, the propensity of the contaminants to attach to the paper machine wire and other surfaces during processing can be significantly reduced.
Previous methods of reducing the tack of contaminants include both chemical and mechanical treatments of process streams. Some methods focus on the use of repulpable or recyclable adhesives. More common methods include the use of chemical additives to modify or de-tack the contaminants. Often, the chemical additives comprise minerals such as talc. These minerals and surface-active chemicals attach to the surface of the contaminants, thereby altering the surface properties of the contaminants to reduce tack. Unfortunately, however, the use of minerals in detackification presents numerous disadvantages, including a loss of effectiveness when exposed to shear and other operational and product-quality problems. In other approaches, certain polymers and enzymes have been used in efforts to reduce the tack of contaminants; however, their high cost and limited effectiveness make them less desirable.
Mechanical methods for controlling contaminants include dispersion, screening, and cleaning. Dispersion is used to break up the contaminants into smaller and smaller particles until they are invisible in the final product. Unfortunately, even the presence of “invisible” contaminants fails to eliminate sticking of adjacent layers when the product is wound, and these contaminants still can greatly diminish the overall appearance of a product.
Screens and centrifugal cleaners also may be used to remove stickies, pitch, and debris from the fiber stream. Screens physically separate the fiber from the contaminants based on the size and shape differences of the contaminants and the holes or slots in the screen. However, the screens' inability to remove contaminants that are smaller than the screen holes or deformable enough to pass through the screen holes limits the effectiveness of the screens in separating the fiber from the contaminants. Centrifugal cleaners separate the fiber from the contaminants based on the different specific gravities of the contaminants and fibers. Separation is poor, however, when the specific gravities of the contaminants and fibers are similar.
Another method for reducing contaminant tack comprises electrohydraulic discharge, as described for example, in U.S. Pat. No. 6,521,134 to Corcoran et al. and U.S. Pat. No. 6,572,733 to Banerjee et al. This method requires, however, an additional piece of capital equipment with its own operating cost and can substantially increase the cost of processing wastepaper.
Several of the foregoing approaches are described in U.S. Pat. No. 4,781,794 to Moreland, U.S. Pat. No. 6,977,027 to Sharma et al., and U.S. Pat. No. 4,956,051 to Moreland.
While the various different techniques and efforts described above offer some relief to the problems caused by contaminants, contaminant tack still results in substantial operational downtime and a degradation of product quality. Furthermore, this problem is exacerbated by the rising costs caused by the increasing demand for recycled paper, forcing many mills to use lower-grade recycled furnish, which may contain higher levels of contaminants and cause more significant problems in processing. Accordingly, a continuing need exists for a simple and inexpensive method to reduce the tack of contaminants.